1. Technical Field of the Invention
The present invention relates generally to an automotive power supply system, and more particularly to such a system designed to ensure the stability in charging a plurality of storage batteries installed in an automobile.
2. Background Art
In recent years, automobiles have been computerized and equipped with various electronics and electrically-operated accessories or devices, thus resulting in an increased electrical power consumption. In order to ensure the stability in supplying required electric power, there have been proposed use of two storage batteries in automobiles. For example, WO 03/004315, U.S. Pat. No. 6,765,312 (WO 01/36232), and Japanese Patent No. 2508767 disclose dual battery vehicles.
It is advisable that typical automobiles have two storage batteries mounted in different locations in light of vehicle weight balance or ease of replacement of the batteries. For instance, one of the batteries may be mounted in an engine compartment, while the other may be mounted in a trunk.
The batteries are usually connected to an electric generator (i.e., an alternator) so that they are charged by electrical energy produced by the generator. The generator is usually installed in the engine compartment and driven by an engine of the automobile. Therefore, one of the batteries in the engine compartment is located closer to the generator, while the other mounted in, for example, the trunk is located far away from the generator. This results in a difference in impedance between power supply cables connecting the generator with the batteries, which leads to a difficulty in charging both the batteries to a desired energy simultaneously. In order to avoid this problem, the above-listed WO 03/004315 teaches use of a DC—DC converter which is interposed between the generator and the batteries to compensate for a voltage drop of the batteries, thereby enabling the batteries to be charged to a desired energy simultaneously.
However, modern automotive electric generators are required to have an increased rated capacity more than 100 A. DC—DC converters suited for such a large amount of power are very expensive. It may be proposed to install two generators in the automobile to charge the batteries independently, but it is difficult due to spatial restrictions.